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copper wire to the ring of a retort stand, and let the open end
dip into lime or baryta water. Heat the mixture gently with a
small flame. The gas which bubbles through the lime water
turns it milky. Moisture will also appear on the sides of the
tube, which, provided that the copper oxide has been thoroughly
dried beforehand, indicates the presence of hydrogen in the
compound. Gases, or volatile substances like ether and
alcohol, cannot, of course, be examined .in this way ; but
an apparatus must be arranged so that the gas or vapour is
made to pass over a layer of red hot copper oxide and then
through the lime water.
Nitrogen.óMany organic nitrogen compounds when
strongly heated with soda-lime give off their nitrogen in the
form of ammonia. Grind up a fragment of cheese or a few
crystals of urea with 5 to 6 times its weight of soda-lime, pour
the mixture into a small test-tube (preferably of hard glass) and
cover it with an equally thick layer of soda-lime. Heat strongly,
beginning at the top layer. Ammonia is evolved and can be
detected by the smell, or by holding a piece of moistened
red litmus paper at the mouth of the tube. When nitrogen is
present in direct combination with oxygen, as in the nitro- and
azoxy-compounds, ammonia is not evolved. The following
general method is applicable to all compounds and is there-
fore more reliable. The compound is heated with metallic
potassium or sodium when potassium or sodium cyanide
is formed. The subsequent test is the same as for cyanides.
Pour' about 10 c.c. of distilled water into a small beaker.
Place a fragment of the substance in a small test-tube along
with a piece of metallic potassium or sodium the size of
a coffee bean, and heat them at first gently until the re-
action subsides, and then strongly until the glass is nearly
red-hot. Then place the hot end of the tube in the small
beaker of water. The glass crumbles away, and any residual
potassium is decomposed with a bright flash, all the cyanide
rapidly goes into solution, whilst a quantity of carbon remains
suspended in the liquid. Filter through a small filter into a test-
tube. Add to the clear solution a few drops of ferrous
sulphate solution, and a drop of ferric chloride, boi'l up for
a minute, cool under the tap, and acidify with dilute hydro-
chloric acid. A precipitate of Prussian blue indicates the